docs: def add_composition(self, composition):
          """Add a composition to the suite.
  
          Raise an UnexpectedObjectError when the supplied argument is not a
          Composition object.
          """
          if not hasattr(composition, 'tracks'):
              raise UnexpectedObjectError("Object '%s' not expected. Expecting "
                      "a mingus.containers.Composition object." % composition)
          self.compositions.append(composition)
          return self